Since Uber and Grab landed here in 2013, the number of cars offering taxi-like services has grown from zero to more than 40,000 - eclipsing a taxi population of around 25,700.

Indeed, studies in US cities such as New York, San Francisco and Seattle suggest that Uber and its rival Lyft have contributed to worsening congestion. Both firms refute these observations.

I am a sceptic of the notion that private-hire vehicles are part of the sharing economy.Singapore University of Social Sciences transport researcher Park Byung Joon

Transport experts say it is hard to pinpoint the causes of congestion, as there are many - from roadworks to weather to rail breakdowns.

Yet, they are not surprised that private-hire cars are clocking such high mileages.

Singapore University of Social Sciences transport researcher Park Byung Joon said: "I am a sceptic of the notion that private-hire vehicles are part of the sharing economy. There is no sharing as it is a paid service."

National University of Singapore transport researcher Lee Der-Horng said: "I do notice the surge of private-hire vehicles has caused some localised congestion at places like shopping malls and office buildings."

He said it may be necessary for building owners and the regulator to come up with pick-up and drop-off points for such vehicles, "just like taxi stands".

Some motorists believe private-hire vehicles contribute to jams.

"Since the car population has fallen, I expect traffic to improve. But I find congestion has worsened in recent years," said businessman Desmond Koh, 58.

Meanwhile, used car traders say it is difficult for them to buy rental or private-hire vehicles because of their high mileage.

"How to resell? Nobody would want because of the high mileage," said one who requested anonymity because his firm supplies cars to Uber.

According to Chinese daily Lianhe Zaobao, Uber's Lion City Rentals sold up to 1,000 cars in the last two months - some as new as a year old.

Asked why it was off-loading so many cars, an Uber spokesman said: "As part of its regular operations, Lion City Rentals regularly refreshes its fleet inventory, so that hirers can choose more of the models they prefer."